EGW writes that the investigative judgement began in 1844 and ends before Christ's return (she quotes several verses in the GC Chapter - Facing Life's Record). She invites us to look into the meaning of Hebrews 6-9, Exodus, Leviticus 16 (Day of Atonement, Christ's intercession in the heavenly sanctuary), Ezekiel, Jeremiah, and several NT books (Acts, Mark, Corinthians, I Peter...).
Judgement is a complex matter and does not begin and end all at once. The Bible depicts several judgement scenes.
1. First, according to prophecy beginning in 1844, Christ enters the Most Holy Place in the heavenly sanctuary (the Millerites had misinterpreted this to mean the Second Coming - the Great Disappointment of 1844). Christ begins with reviewing the lives of the dead, everyone who has ever lived and confessed Christ (including Adam).
2. The judgement then moves to the living (Ezekiel and Peter describe this).
Scenes 1 and 2 are the investigative judgement in which people's actions, thoughts and decisions are reviewed against the high standard of God's law, as well as sins confessed and forsaken by the atoning blood of Christ, determine who will be saints in heaven.
3. Christ comes with "My reward" at the Second Coming to take the saints home. Judgement has been done and the verdict declared in favor of the saints and against the wicked, both living and dead. Christ comes to wake up and take home the righteous dead as well as the righteous living to be with Him 1,000 yrs. The wicked dead are killed by His bright appearing.(I Cor 15:50; I Thess 4:15--).
By this time, Satan and his followers have had no chance to defend themselves in open court.
4. During the millennium, the saints also do judgement, this time not to decide who will become saints but to review the books and "confirm" or vindicate God's righteous judgement against the dragon, the beast, the false prophet and the wicked. They also vindicate God's judgement in declaring them His saints. Satan is bound 1,000 years in a desolate earth at that time and cannot access the wicked dead to tempt them, nor can he reach the saints who are in heaven.
5. After 1,000 years, the wicked dead are raised in the second resurrection to receive their judgement along with Satan, the beast and the false prophet (whose judgement and fate will have been decided by the time of the Second Coming). This is the "open court" scene in which Satan and the wicked admit that God is indeed righteous and His judgement is fair.